The timing of an internal combustion engine must be accurately controlled so that emissions are minimized and so that the engine runs at peak efficiency. In compression type or diesel engines, ignition occurs upon injection of fuel into a cylinder containing air which has been compressed by a piston which is movable in the cylinder. The "timing" of such an engine is defined as the time at which a fuel injector is operated to inject fuel into the cylinder relative to the time at which the piston reaches a position known as "top dead center" in the cylinder which is reached at the end of the piston stroke. In past diesel engines, the fuel injectors have been of the mechanical type which are controlled by a cam shaft which is geared to the crankshaft of the engine. In recent years, however, electronic or solenoid controlled fuel injectors have been adopted for use and which are operated by an engine control and a solenoid driver circuit. Such an arrangement is disclosed in Pflederer, U.S. Pat. No. 4,604,675, entitled "Fuel Injection Solenoid Driver Circuit" and assigned to the assignee of the instant application. A somewhat modified solenoid driver circuit is disclosed in Grembowicz, et al., U.S. patent application Ser. No. 07/260,241, filed Oct. 20, 1988, entitled "Driver Circuit For Solenoid Operated Fuel Injectors" (Caterpillar Case No. 88-264) and assigned to the assignee of the instant application. Both of these applications are expressly incorporated by reference herein.
In diesel engines which utilize electronic fuel injectors, some provision must be made for sensing the position of the pistons within the cylinders relative to top dead center (TDC). Luebbering, U.S. patent application Ser. No. 07/078,728, filed July 28, 1987, entitled "Apparatus For Determining the Speed, Angular Position and Direction of Rotation of a Rotatable Shaft" and assigned to the assignee of the instant application (Caterpillar Case No. 86-136), the disclosure of which is also hereby incorporated by reference, discloses the use of a magnetic pickup disposed adjacent a toothed gear or wheel coupled to a crankshaft or cam shaft of an engine. The magnetic pickup develops a series of pulses which are representative of the positions of the pistons in the cylinders. This information is used to accurately control the actuation of the fuel injectors.
It has been found that manufacturing and assembly tolerances can cause the piston position indicated by the magnetic pickup to be shifted or offset relative to the actual position of the piston in the cylinder. This shift, if uncorrected, causes a loss of timing accuracy, resulting in poor engine performance and increased emissions.